Processes that enable patterned deposition of materials onto a substrate have a number of applications, especially in the electronic microcircuit industry. Microfabrication of electronic circuits relies on the ability to create multi-layer patterns of numerous functional materials, with varying electrical properties. The technologies used for creating these multi-layer patterns may be additive, subtractive, or a combination of the two. Additive technologies deposit the functional material on the substrate in the desired pattern, i.e., the pattern is generated directly on the substrate during the deposition/layering process. Subtractive processes, on the other hand, first create a continuous layer of the functional material on the substrate. The desired pattern is then subsequently created by the selective removal of functional material from the deposited layer, i.e., the pattern is created subsequent to the deposition/layering process. A detailed description of various patterned deposition/layering processes used in the microfabrication industry may be found in “The Physics of Micro/Nano-Fabrication” by Ivor Brodie and Julius J. Murray, Plenum Press, N.Y., 1992.
Traditional micro-fabrication processes utilize either or both the additive and subtractive processes depending upon the specific application, and are generally carried out in a high vacuum (low-pressure) environment. The high vacuum process generally involves the evaporation of functional material by heating or by ion bombardment followed by deposition onto the substrate by condensation or by a chemical reaction. In these deposition processes, the functional material is required to be thermally stable or to have a thermally stable precursor that can generate the functional material on the substrate by a chemical reaction. As skilled artisans will appreciate, these processes are not useful in generating patterned layers of thermally unstable materials.
Further, those skilled in the art will appreciate that it is common to use a mask technique for patterned deposition. Typically, the mask employed for patterning on a planar substrate surface is a photoresist material. However, when the surface is nonplanar, difficulties can be encountered in depositing and cleaning off the photoresist material, necessitating the use of shadow masks or stencils. For example, U.S. Pat. No. 4,218,532 titled “Photolithographic Technique For Depositing Thin Films,” issued Aug. 19, 1980 to Dunkleberger discloses a method for patterned deposition of thin films of metals, such as lead alloys, by vacuum evaporation onto a substrate through openings in a mask fabricated with a predetermined pattern. A shortcoming of this development is that it cannot be used for the patterned deposition of thermally unstable materials since these are not suitable for vacuum evaporation.
In U.S. Pat. No. 4,013,502 titled “Stencil Process For High Resolution Pattern Replication,” issued Mar. 22, 1977 to Staples, a process for obtaining high-resolution pattern replication using stencils is disclosed. The stencil in Staples is a mask effecting molecular beam deposition of thin films onto a substrate through openings in the stencil. In this deposition process, the molecular beam source is an electron-beam evaporator. Much like the Dunkleberger development, a shortcoming of Staples' technology is that it cannot be used for patterned deposition of thermally unstable materials that are not suitable for evaporation using an electron beam evaporator.
Furthermore, it is well known that patterned deposition of thermally unstable materials on substrates may be achieved by liquid phase processes such as electroplating, electrophoresis, sedimentation, or spin coating but these processes are system specific. For example, in the case of electroplating, it is necessary that an electrochemically active solution of the functional material precursor is available. In the case of sedimentation and spin coating, a stable colloidal dispersion is necessary. In the case of electrophoresis, it is also necessary that the stable colloidal dispersion be charged. Microfabrication of multi-layer structures usually requires multiple stages, necessitating the complete removal of residual liquids/solvents at the end of every stage, which can be very energy, time, and cost intensive. Further, many of these liquid-based processes require the use of non-aqueous liquids/solvents; which are hazardous to health and the disposal of which can be prohibitively expensive. For example, in U.S. Pat. No. 5,545,307 titled “Process For Patterned Electroplating,” issued Aug. 13, 1996 to Doss et al., a process is disclosed for patterned electroplating of metals onto a substrate 14 through a mask. The Doss et al. process, however, has at least two major shortcomings. First, it is only applicable to materials that have electrochemically active precursors. Second, it uses an aqueous electroplating bath for the process that requires the coated substrate be cleaned and then dried at the end of the coating process.
Moreover, it is well known that to eliminate the need for potentially harmful solvents that need drying, it is possible to use environmental and health-benign supercritical fluids such as carbon dioxide as solvents. For example, in U.S. Pat. No. 4,737,384 titled “Deposition Of Thin Films Using Supercritical Fluids,” issued Apr. 12, 1988 to Murthy et al., a process is disclosed for depositing thin films of materials that are soluble in supercritical fluids onto a substrate. Murthy et al. include the steps of exposing a substrate at supercritical temperatures and pressures to a solution comprising a metal or polymer dissolved in water or a non-polar organic solvent. The metal or polymer is substantially insoluble in the solvent under sub-critical conditions and is substantially soluble in the solvent under supercritical conditions. Reducing the pressure alone, or temperature and pressure together, to sub-critical values cause the deposition of a thin coating of the metal or polymer onto the substrate. Nonetheless, a shortcoming of the process of Murthy et al. is its limited applicability to materials that can be dissolved in compressed fluids, severely limiting the choice of materials that can be deposited on a substrate using this technology. Another shortcoming of the process of Murthy et al. is that it does not teach a process for the patterned deposition of functional materials.
In U.S. Pat. No. 4,582,731 titled “Supercritical Fluid Molecular Spray Film Deposition and Powder Formation,” issued Apr. 15, 1986 to Smith, and U.S. Pat. No. 4,734,227 titled “Method Of Making Supercritical Fluid Molecular Spray Films, Powder And Fibers,” issued Mar. 29, 1988 to Smith, independent processes are disclosed for producing solid films on a substrate by dissolving a solid material into supercritical fluid solution at an elevated pressure. In both cases, the supercritical fluid solution is then rapidly expanded in a region of relatively low pressure through a heated nozzle having a relatively short orifice. Both of the aforementioned Smith processes have similar shortcomings to those indicated above, i.e., they are only applicable to materials that are soluble in compressed fluids and do not teach a process for patterned deposition.
Therefore, a need persists in the art for a patterned deposition system that permits the patterned deposition of thermally unstable/labile materials and that eliminates the use of expensive and both environmentally and human health-hazardous solvents. A further need exists for a patterned deposition system that eliminates the need for post-deposition drying for solvent-elimination. Moreover, there is an additional need for a patterned deposition technique that is applicable for a wide range of functional materials and that is not limited by specific properties of the functional materials.